12 April, 2014

If you have
ever been asked, which sense is the most important sense to sharks? Keep
reading…

The good people at Mote Marine Lab have done it again. They seem to always crank out the good stuff, but this study is simply exemplary. Here’s an
overly simplified version of the bits I liked the most – for all the bits, go
read it yourself!

They collected several individuals of three
species of shark - blacktips C.limbatus,
bonnetheads S.tiburo, and nurses G.Cirratum. They also collected live prey items from the
diets of each (blacktip+nurse = pinfish L.rhomboides
and bonnethead = pink shrimp F.duorarum). Each species’ general 'norm' of how they go
about detecting, tracking, orientating, striking and capturing their prey was
established. Researchers then blocked some of
four senses (smell, vision, electroreception, lateral line) one at a time or in
combination and observed how the individuals adapted to these changes and
shifted their behaviours from the established ‘norm’ of all senses firing.

When it
comes to the initial detection of prey, the
sense of smell reined king for all three species. While blacktips and bonnetheads would still capture
the prey when they eventually came into visual contact with it, they did not
detect and track the prey from a distance as they would normally. For nurse sharks, sense of smell was an
absolute necessity. With their nares
blocked, nurse sharks sat at the bottom of the tank and failed to feed
entirely. When smell and vision were blocked, blacktips and bonnethead also failed to
feed. This, of course, makes sense. Nurse sharks primarily hunt at night or hunt
prey animals hidden from view in reefs, whereas blacktips and bonnetheads have
light available to them. So guess who
performed the best when vision and lateral line were blocked? Yep, the nurse shark. Who did the worst? Bonnetheads.

The sense of
electroreception was linked with successfully capturing prey. When blocked, the sharks could still detect,
track, orientate and strike at the prey, but would simply forget to open their
jaws in time to capture it, even if the prey actually touched the shark!

Source. A. A bonnethead,Sphyrna tiburo, with all senses intact opens the mouth to capture shrimp using ram-biting. B. The same bonnethead fails to open the mouth when electroreception is blocked and misses the shrimp, despite making tactile contact with the prey. doi:10.1371/journal.pone.0093036.g006

Vision and
lateral line seemed to go hand in hand: “…animals
with simultaneous vision and lateral line blocks did not orient or strike, even
when they were within electrosensory prey detection range.” Thus, “these results suggest that sharks do
not recognize electrical cues alone and prey, but require an additional visual
or olfactory cue.”

In conclusion:

Our results demonstrate that sharks are capable of attending to multiple sensory cues simultaneously, switching sensory modalities in a hierarchical fashion as they approach their prey, and substituting alternate sensory cues, when necessary, to accomplish behavioural tasks. This flexibility in behaviour suggests that sharks are well adapted to success, even in the face of a changing environment and evolutionary advancements in prey defenses including chemical, visual, and mechanical camouflage. Gardiner et al. 2014

The bottom
line? Sharks are not the super simple
one-sensory fish that some may want you to believe that they are. Elegant behavioural research, bravo!

It would be
very interesting, but impossible, to do a similar study on large migratory sharks to see
what senses they may use to navigate over long (say, Dyer Island to Mozambique) distances, but alas...

08 April, 2014

Could be shark, tuna, ray, or anything that is counter-shaded that hangs out along the surf zone. Looks too chill to be a dolphin, and if that is a pectoral fin it is long and has a black tip. Or it could just be another damn surfer (thanks David!).

One thing's for sure, Kelly'I would be honored to be eaten by a shark'Slater has zero f*cks to give.

03 April, 2014

Shark repellents
seem to creep up in the
news once every three months or so.
Some make me laugh, some show
promise, and others straight up vex me. The above wetsuits fall into the latter because
they can never work, definitely not for the reasons the
sellers are trying to convince you, at least. So let’s examine the pseuodoscience behind
this, delve into the true science, and see how these suits measure up.

The concept
behind the wetsuit is that the coloration on them mimics the colouration of sea snakes, and that sharks will avoid sea snakes (or anything black and white striped) because they are poisonous. Therefore
any shark would recognize the coloration of the sea snake on your wetsuit as poisonous
and avoid you – thus preventing shark attacks.

This idea is
based on Batesian mimicry,
which is when non-harmful species mimic the 'warning'coloration of a harmful
species in order to reduce their predation risk from a common predator. The classic example of this is the monarch
butterfly and the viceroy:

The monarch
(on the right) is poisonous. If a
predator – like a Blue Jay – consumes the monarch, it will be instantly sick. This instantaneous negative reaction is key,
because the Blue Jay immediately associates being ill with the consumption of
that orange and black coloured prey. If
the negative reaction is delayed, say a few minutes later after the Blue Jay
has eaten another beetle, the Blue Jay will not learn that it was the monarch
that caused the reaction. So, the
monarch has taught the Blue Jay to avoid orange and black coloured butterflies.

The non-harmful
viceroy, through the random luck of mutations and natural selection, mimics the
coloration of the monarch butterfly.
This is because these warning colours are highly advantageous, thus
increasing the viceroy’s fitness.

c.As for the sharks that don’t eat poisonous sea
snakes, they would have not learned aversion to the colouration now would they?

Secondly, the
effectiveness of warning colouration is measured
on the species level (less monarchs are eaten overall – the species survives)
not the individual level. So even if sharks did avoid anything with black and white stripes, that doesn't necessarily mean you would be 100% safe. This is because learned behaviours often have to be relearned. The Blue Jay in the above example won’t eat
another monarch for quite some time, but next season he may very well have
forgotten and experience another negative reaction to orange and black
butterflies to remind him that they should be avoided (unless he eats a viceroy first and has no negative reaction... see how this goes?).

So, applying what we now know from above, what are the odds that a non-Tiger shark will approach
you in the water combined with 2)
the individual shark approaching you has recently
eaten a poisonous snake of black and white colouration and 3) it was ill immediately after eating that snake, therefore
has learned an aversion?? ZERO.

The second
hypothesis, and style of wetsuit, concerns cryptic colouration. They say,

“So while a shark may locate its prey through a number of different means, it is less likely to attack if the target cannot be seen.”

Yes, because
sharks only attempt to eat
what they can see. This is why sharks only eat prey in clear water and high light levels... wait, what? Bottom line is, if you are in the water near a shark, it has already noticed you. Several variables afterwards will determine whether or not it comes in for a closer inspection, and none of those variables concern what color your wetsuit is.

24 March, 2014

Unfortunately, another person has been fatally wounded by a shark at Port St. John's Second Beach - the NSRI reports. He was a 72-year-old Australian at the beach as part of a tourist group.

What is the answer to mitigate shark/human interactions in what is a wild part of South Africa's coastline? There are quite a few groups interested in doing projects in this area, but they are all hampered by the location (and fairly shite Eastern Cape government officials). Here's hoping a local group takes charge.

14 March, 2014

The Save Our Seas Foundation wants to produce more of the best underwater photojournalists on the planet, and they are putting their money where there mouth is. If you think you've got what it takes to win $2,000 up front and then a three week PAID expedition with SOSF, and then PRESENT your work at an all expenses paid conference.... you need to apply before 30 April 2014!

20 January, 2014

Sorry for the lack of blogging, folks! Although there is much shark madness to discuss, more imperative issues demand my attention. Thank you for your patience and readership, you are not being neglected.

P.S. Just noticed the Megalodon blog slipped past 2500 views, and is still rising! Who knew it would be so popular?